1. Technical Field
The present invention relates to load handling devices for stabilizing the lifting of a load. More particularly, the present invention relates to a cable arrangement and a lifting platform, such as a six-axis range of motion robot having self-balancing means, for the stabilized lifting of a load attached thereto.
2. Background Information
Lifting platforms are well known in the art. Commonly, lifting platforms are attached to cranes, such as overhead tower cranes having a horizontal boom and boom cranes having a diagonal boom. Applications for these lifting platforms include transporting cargo on and off ships, and relocating necessary equipment and materials on a construction site. In order to best understand the present invention, a cartesian coordinate system will be defined such that the Z-axis is in the vertical direction, and the X and Y axes form the horizontal plane. Roll is defined as rotation about the Z-axis; pitch is rotation about the X-axis; and yaw is rotation about the Y-axis.
In typical load transporting applications, a crane will have a single lifting cable. In these applications, the lifting cable is stable only in the Z direction. Under any pressure from the sides, the load will either rotate in roll, pitch and yaw, or will sway in the X and Y directions.
The prior art has long recognized the need to stabilize the load suspended from a single load lifting cable. For example, in U.S. Pat. No. 2,916,162 issued to Gercke, a diagonal boom crane is shown having a single load lifting cable for transporting loads. Gercke is directed to an apparatus for damping the pendulum motions of the load suspended from the lifting cable. The Gercke apparatus comprises a plurality of L-shaped levers which surround the load lifting cable near the top of the boom crane. As the load lifting cable sways, these levers are caused to move, and their movement is sensed by sliding potentiometers. Each lever is attached to a potentiometer, and all potentiometers are attached to a motor which controls the position of the levers. The more the load tends to swing, the more the levers try to suppress the load's swing. Although Gercke may tend to suppress the pendulum motions in the X and Y directions, the Gercke device fails to suppress any load imbalance causing roll, pitch or yaw. Such drawbacks are inherent with single-cable lifting devices.
Other systems have been developed which try to solve the problems inherent in single-cable load lifting arrangements by employing a plurality of cables. For example, in U.S. Pat. No. 3,743,107 issued to Verschoof, a four cable arrangement system is shown for preventing a container load, attached to a container yoke with the yoke suspended from four hoisting cables, from swinging in the horizontal direction. Four cables are used in the Verschoof system: two cables are attached to a common winch and wrap around the container yoke via pulleys, the ends of these two cables being securely attached to the frame which secures the winch; the other two cables are attached to the container yoke in a cross-hatched manner such that the cables are securely attached to the container yoke at one end, and attached to the securing frame via tension devices. The tension devices sense cable slack, due to load imbalance and shifting, and adjust the tension on the respective cables such that the tension on both cables remain equal. Verschoof allows for the hoisting and lowering of the container yoke via the first two cables, while providing for load imbalance in the horizontal plane via the second pair of cables.
Both Gercke and Verschoof are directed to stabilizing loads by sensing any load imbalance through the attachment cables. Other systems, however, are directed to sensing load imbalance at the load attachment platform. For example, U.S. Pat. No. 4,350,254 issued to Noly, herein incorporated by reference, is directed to a load platform suspended from an overhead tower crane by four lifting cables. The lifting platform, a spreader frame for attaching to railroad containers and the like, includes means for adjusting the load along the length of the platform based on imbalance in that direction. Additionally, the lifting platform includes means for rotating the attached load in a 360.degree. angle of rotation and means for tilting the attached load in a slight angle relative to the lifting platform for ease of lifting and/or placement of the load. The four lifting cables which attach the lifting platform to the overhead tower crane are adjusted via a pair of winches, each winch attaching to the opposite pair of cables. Although Noly allows for automatic load imbalance compensation in the direction relative to the length of the lifting platform by moving the load in that direction, Noly does not compensate for load imbalance in the direction relative to the width of the lifting platform by movement of the load. Rather, Noly states that any imbalance along the width of the load platform is compensated by the dual-winch take-up system having the opposite cables attached thereto. Although Noly's use of a dual-winch take-up system compensates for load imbalance in the direction of width, a dual-winch system adds considerable complexity and cost to load handling systems. Additionally, should a load imbalance be substantial in the direction of width, the strain and tension on the cables will lead to a serious degradation in the integrity of the cables and the winch system.